Single-conversion UPS systems are those in
which, during normal operation, the incoming line is used to provide power to
the critical load either through a transformer or in conjunction with some
series impedance.
Some forms of single-conversion UPS
products are classified as “line interactive”. The single conversion UPS
usually provides a higher operating efficiency at lower cost than the double
conversion UPS at a comparable system reliability. (Note that the recently
revised NEMA PE 1-1993 [B23], identifies the single-conversion system as a
“single-conversion converter.”)
Unlike the double-conversion system, the
incoming line to the single-conversion UPS is not rectified to produce dc power
to provide input to the inverter. The normal ac power is supplied directly to
the critical load through a series inductor or a linear or ferroresonant
transformer.
The normal ac also supplies a small charger
used to maintain the UPS batteries in a fully charged condition. Thus the
battery is only used when inverter requires the battery’s output to supplement
or replace the normal power source. Single-conversion UPSs include the
ferroresonant type, some variants of the tri-port type, line interactive types,
and some SPS designs. A discussion of some of these systems follows.
Line interactive systems
The line interactive UPS is rapidly
achieving prominence in the industry. Lower cost, simplicity of design, and
smaller “package” are some of the advantages of this design.
In the line interactive UPS, line power is
not converted into dc but is fed directly to the critical load through a series
inductor or transformer. Regulation and continuous power to the load is
achieved through use of inverter switching elements in combination with
inverter magnetic components such as inductors, linear transformers, or
ferroresonant transformers.
The term “line interactive UPS” derives
from the fact that the inverter interacts with the line to buck, boost, or
replace incoming power as needed to maintain constant voltage to the critical
load. In the absence of line power, the line interactive UPS provides the total
power to the critical load.
Some line interactive products exhibit a
break in power to the critical load during line-to-battery or battery-to-line
transfers (this type of system would be an SPS). Other designs are capable of
producing true unbroken power to the critical load. Line interactive UPSs are
available as single-phase or three-phase units.
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